I don't think napping in the carrier, pram or car will put you ten steps back. LOs do get into habits of where to sleep so if you consistently have one nap at home in the cot I think LO will always be prepared to take that nap at home in the cot. Equally if you always have one nap in the carrier per day LO will come to expect that. And my personal opinion is that habits of always having a nap here or there have positive and negative points.
My DS got so used to napping in his cot he was pretty impossible to AP and I really struggled with this for a time, he totally refused to sleep in his pram so trips out became a nightmare. He would sleep in his carrier so long as I was on the move and not just pacing - so I could walk for miles but I couldn't AP him at a luncheon or party! And eventually, unless you are really strong, they do start to feel pretty heavy in the carrier.
We also did lots of afternoon naps in the car on the way to baby groups and on the way home again. For months I felt trapped in the car...but he got his sleep and it meant we got to go out. Never has any of it effected his ability to independently sleep in his own bed at home, in fact that's where he wants to be. He will now (finally) sleep in his pram but again we have to be on the move and not just rocking him.
